The invention relates to an alloy on the basis of the intermetallic phase .gamma.-TiAl and to a method of making such an alloy.
Such an alloy consists mostly of the .gamma.-TiAl phase and is known, for example from the publication "N. Zheng, W. Fischer, H. Grubmeier, V. Shemet, W. J. Quadakkers--`THE SIGNIFICANACE OF SUB-SURFACE DEPLETION LAYER COMPOSITION FOR THE OXIDATION BEHAVIOUR OF .gamma.-TITANIUM, Script Metall. et Mater. 33(1995) 47-53".
The alloy finds more and more use as a construction material for high temperature components. When compared with conventional materials, .gamma.-TiAl alloys offer great advantages in building components for which a combination of high strength and low density is required such as in stationary gas turbines and in jet propulsion engines.
An obstacle preventing the widespread introduction of the .gamma.-TiAl alloys is their insufficient oxidation resistance at temperatures above about 700.degree. C. The reason herefor is that, inspite of a high Al content of 42 to 55% Al (generally 48-50%), the .gamma.-TiAl alloys do not form slowly growing protective Al.sub.2 O.sub.3 layers during high temperature applications. Rather TiO.sub.2 rich mixed oxide layers with high growth rates are formed resulting in relatively high wall thickness losses of the components which is unacceptable for long term applications.
It has become known in the meantime that no protective Al.sub.2 O.sub.3 layer is formed because of the presence of .alpha..sub.2 -Ti.sub.3 Al in the depletion zone immediately beneath the surface oxide layer. The .alpha..sub.2 phase has a very high oxygen solubility (.apprxeq.20 At.- %), so that the Al oxide occurs as internal oxidation rather than forming a protective surface layer.
Several authors have shown that the growth rate of the heterogeneous, TiO.sub.2 rich oxide layers can be reduced by additions to the alloy of Mo, Cr, and particularly Nb(2-5 At.- %). However, inspite of this improvement, the oxidation resistance of these ternary or quaternary alloys is still insufficient for use at temperatures above about 800.degree. C.
It has been known for some time that, in the initial stages of the oxidation of .gamma.-TiAl alloys, protective layers on the basis of Al.sub.2 O.sub.3 can actually be formed. However, these protective layers, particularly in N.sub.2 containing process gases (for example air), have no long-term stability. Already after relatively short operating periods (of for example .ltoreq.100 h, depending on the temperature), a conversion to rapidly growing TiO.sub.2 -rich layers occurs.
From the printed publication, "N. Zheng, et al." referred to earlier, it is known that the formation of an initially protective Al.sub.2 O.sub.3 layer is caused by a different composition of the depletion zone just below the oxidation layer. While there is .alpha..sub.2 -Ti.sub.3 Al beneath the non-protective layer, as mentioned above, the depletion zone below the initial Al.sub.2 O.sub.3 layer consists of a previously unknown ternary Ti--Al--O phase of an approximate composition Ti.sub.3 Al.sub.3 O.sub.2 (designated a Z-phase) which possesses a cubic structure. Thermodynamic and kinetic considerations show that Al.sub.2 O.sub.3 is stable in equilibrium with the Z-phase, since the last mentioned has a greater Al activity and a lower oxygen solubility than .alpha..sub.2. Because of the structural properties of the Z-phase, which have in the meantime been determined, methods for the long term stabilization of this phase in the depletion zone below the protective Al.sub.2 O.sub.3 based layer can be derived.
This knowledge provides the possibility of developing .gamma.-TiAl alloys with greatly improved oxidation resistance.